What is that spicy green sauce that you always find in mexican restaurants?

I go to 2 different authentic mexican restaurants that aren't fast food like taco bell but aren't fancy sit down restaurants either. They both offer the option of getting little cups of hot sauce that are either red or green. I love the green stuff but I'm just not sure what it is.What is that spicy green sauce that you always find in mexican restaurants?
it is salsa verde...
It's called salsa verde (green sauce) and is very simple to make. For fresh sauce use 5 - 6 tomates verdes also known as tomatillos in the USA, 1 slice of onion, 1 - 6 serrano peppers (depending on how hot you like your sauce) and 1 sprig of cilantro (optional). 1 oz water Liquefy all ingredients in blender. if it seems too thick add a little more water - the tomatillos are actually a thickening agent. Add a pinch of salt.

For cooked salsa place tomatillos, onion and serrano(s) in one quart of boiling water until the color of the tomatillos becomes more olive green. Add cilantro at end of boil (optional). Liquefy vegetables in blender with 1-2 oz. of water from boil. Add a pinch of salt. Enjoy!

Edit: oops I forgot! Another variation is to make the first recipe above, add juice of one lime and one medium avocado. Mash the avocado to a pulp and mix with the salsa. This is NOT to be confused with guacamole in which avocado is the main ingredient.What is that spicy green sauce that you always find in mexican restaurants?
Salsa Verde. Normally made with tomatillos, green chili and perhaps minced onion and a little garlic. Sometimes cumin. All depends upon where you go and what their recipe is. Its easy to make.
Salsa Verde. Its known as the green salsa. Pretty good to.
Salsa Verde
salsa verde
verde
guacamole
i mean.. salsa verde.. obvs